Not Applicable
1. Field of the Invention
This invention relates to apparatus and method for determining the rate of flow of a liquid by measuring an electrical potential difference developed in the liquid as it moves through a magnetic field. The invention particularly addresses the inherent problems related to insertion flow probes.
2. Background Information
In a magnetic flow sensor an electrical potential difference developed in an electrically conductive liquid is detected by a pair of electrodes contacting the liquid which are spaced apart from each other along a line that is generally orthogonal to both the direction in which the flow is measured and a magnetic field produced by an electromagnet. This arrangement is used in most commercially available full bore sensors where the electrodes are typically mounted in electrically insulating liners inside of metal shells, and at the bottom of insertion probe sensors.
Typical full bore magnetic flow sensors provide high precision measurement of flow rate but are bulky and expensive to purchase, install and remove. Typical magnetic probe flow sensors have relatively lower measurement precision and also lack the installation ease of other insertion-type probe sensors. For example, the flow transition around such a probe is abrupt because the bottom of the sensor is the flat end of a relatively stubby cylinder. This can produce turbulence and an uncertain path of the liquid flow producing the electrode signals, ultimately leading to a reduction of measurement precision. Because flow rate detection occurs at only one location in the flow profile, variations in the flow rate at other locations in the flow profile can also affect the measurement accuracy on a volumetric basis. When a probe sensor is mounted in pipes which are not much larger than the probe itself, the probe creates a pressure drop and leads to flow measurement uncertainties which may not be tolerable in some applications. Furthermore, because the signal magnitude depends, in part, on the size of the electromagnet providing the magnetic flux and the distance between the electrodes that is engaged by that flux, probe mechanical mounting diameters must generally be large compared to other types of insertion type sensors in order to produce sufficient electrical signal at the electrodes. Other problems associated with prior art magnetic probe flow sensors include a requirement for a larger opening through a pipe, and the greater pressure that the liquid exerts on the probe. This pressure acts to push the probe out of the pipe and leads to a larger hot tap insertion force because of the larger supporting stem diameter. The stem diameters must then be relatively large to prevent stem deflection at the higher flow rates. And finally, prior art sensors, make relatively inefficient use of the magnetic field and generally require substantially more electrical operating power than do other types of probe sensors.
It is therefore an object to provide a magnetic flow sensing probe which offers significant improvement over the prior art just described.
It is a further object to adapt the improvements of the probe sensor configuration to the inline sensor configuration.
The above and other objects of the invention are attained by magnetic flow sensors in accordance with various preferred embodiments of the present invention. In preferred embodiments, the magnetic axis (i.e., the line extending from the south to the north pole) of an electromagnet is oriented generally perpendicular to a direction of flow of a conductive liquid. As is known in the magnetic flow metering art, the flux from a magnet arranged in this fashion generates in the liquid a voltage difference proportional to the flow rate of the liquid. In various embodiments of this invention this voltage difference is sensed by a sensing head comprising, in addition to the magnet, a pair of electrodes (which preferably have the same size and shape and are made of the same material) spaced apart from each other along a separation line that is generally orthogonal to both a direction of flow and the magnetic axis. These electrodes may be located in a circular shroud.
The voltage indicative of flow rate is measured between the corresponding two electrodes of a pair when the associated magnetic flux is present and stable, as is known in the magnetic flow metering art. This may consist of a cyclic processing procedure including the measurement and storage of first a first electrode difference potential when no magnetic field is present, followed by a similar measurement with the field present. In this arrangement, the difference between the two measurements is representative of the liquid flow rate.
In a preferred embodiment of the present invention the flow passage is defined by a cylindrical shroud and the electromagnet is located close to the region of fluid flow being sensed by the electrodes so that the flux which it produces is more effectively used to generate the flow related potentials in the liquid. This close spacing is facilitated by routing the signal leads from the electrodes directly through a hole in the core of the electromagnet instead of running outside of the electromagnet, as is done in the prior art. The core, or at least a portion thereof, is preferably electrically conductive and grounded so that the leads are electrostatically shielded from the relatively high potentials existing on the outside of the coil of the electromagnet. This enables the signal amplifiers to experience lower voltage transients before the magnetic flux stabilizes. The amplifiers can recover faster and accurately detect the electrode voltages sooner after the flux stabilizes, thereby reducing electrical power consumption. It has been confirmed in practical examples that the magnetically induced voltages in the leads are common mode voltages and of a low enough magnitude to be well tolerated by the signal amplifiers. These voltages are inherently equal as they are derived from conductive paths that are easily made mechanically the same and symmetrically located around the core. This is much more difficult to achieve if the leads are instead routed on the outside of the coil where both magnetic and electrostatic induced voltages have to be coped with. Routing the leads through the core also enables the outside of the housing containing the electromagnet to be smaller.
The obstruction free shroud enables a relatively smooth and predictable passage of flow to be realized improving the precision of the liquid flow rate measurement. The improved magnetic flux utilization and electrode signal routing enables a smaller magnet and its housing to be used, thereby reducing its physical dimensions and that of the supporting stem. This reduces the flow obstruction and permits the supporting stem diameter to be made relatively small.
In another preferred embodiment of the present invention, an electromagnet similar to the one described above has a second, similarly configured flow shroud and electrode pair located on its opposite pole. The signals from both sets of electrodes may be directly electrically summed or otherwise combined, or may be passed through individual electronic processing circuits where they may be examined individually, compared and combined as desired. When the signals are directly combined in series aiding the net output signal approaches twice that from a single electrode pair. If desired, the shroud, magnet size and overall mechanical structure can be substantially reduced and still provide the same signal sensitivity as in the single channel embodiment described previously while being less obstructive of the flow path and easier to insert under pressure. The utilization of the previously unused magnetic field for the second shroud enables higher energy efficiency to be achieved as most of the energy used by magnetic flow sensors is for generating the magnetic field. Furthermore, The flow sensing at the two spaced apart locations provides a better sample of the overall flow rate for better volumetric accuracy. Using the same configuration more than one set of such dual sensing heads may be mounted in line one after the other to sense the flow rate over a large linear distance.
The present invention, although developed for insertion probes, could be applied to inline flow sensors in some applications, when energy efficiency is important as in the case of battery and current loop powered equipment.
Although it is believed that the foregoing recital of features and advantages may be of use to one who is skilled in the art and who wishes to learn how to practice the invention, it will be recognized that the foregoing recital is not intended to list all of the features and advantages. Moreover, it may be noted that various embodiments of the invention may provide various combinations of the hereinbefore recited features and advantages of the invention, and that less than all of the recited features and advantages may be provided by some embodiments.